The present invention relates generally to balanced microwave components and deals more particularly with microwave amplifiers having near constant gain over a broad band, processes for making such amplifiers, and other microwave components having desirable transfer functions.
It is presently known to use a single-ended microwave amplifier comprising in series an input matching network, an active gain device, and an output matching network. Typically, the gain of the active device varies with frequency and is greater at lower frequencies than at higher frequencies within the operating band. To compensate for the gain variation of the active device, the input and output matching networks are purposely designed to substantially mismatch the impedances of associated transmission lines at the low frequencies and provide better matching at the higher frequencies. Even though such a matching technique tends to equalize the overall gain of the amplifier, the input and output matching devices cause undesirable reflections at the frequencies of mismatch and a resultant gain ripple in the operating band. The problem is exacerbated if two or more of the single-ended amplifiers are cascaded, because the reflections add to one another in and out of phase between the stages.
It is also presently known to use a balanced configuration of amplifiers, as shown in FIG. 1 and described in more detail below, to mask the reflections. The balanced amplifier includes two identical single-ended amplifiers, as described above, and two identical 3 dB 90.degree. hybrid couplers such as Lange couplers, one preceding and one following the pair of single-ended amplifiers. The Lange couplers attempt to mask reflections created at the inputs to the input matching devices and at the outputs of the output matching devices of the single-ended amplifiers. However, the Lange couplers are not extremely effective in masking the reflections at frequencies at the band edges and at the center of the band, so that the balanced amplifier gain will not be uniform throughout the band.
The presently known balanced amplifiers are manually tuned after manufacture to attempt to flatten the gain. However, the tuning requires skilled labor, is time-consuming and is not 100% effective.
It is also presently known to provide balanced pin-diode attenuators and balanced gain equalizers which are similar in design to the balanced microwave amplifiers described above, except that pin diodes and passive equalizing circuits, respectively, replace the single-ended amplifiers. Such balanced attenuators and equalizers also utilize identical Lange or other 3 dB 90.degree. hybrid couplers at the input and output which cause two peaks in the transfer function of the balanced attenuator and equalizer. Generally, the two peaks are not desirable.
A general object of the present invention is to provide a microwave amplifier having a substantially flat gain over a broad band.
Another object of the present invention is to provide amplifiers of the foregoing type which are cascadable without excessive gain ripple.
Still another object of the present invention is to provide an amplifier of the foregoing type which exhibits an acceptable uniformity in gain with little or no tuning, or a much better uniformity in gain than conventional balanced amplifiers with the same amount of tuning for each.
Yet another object of the present invention is to provide methods for designing amplifiers of the foregoing types.
A further object of the present invention is to improve the transfer function of other balanced microwave components, such as balanced pin-diode attenuators and balanced gain equalizers.